Currently the approach to transferring continuous bit rate services across an ATM (Asynchronous Transfer Mode) network is to use continuous bit rate cell streams. Such streams are transferred through the ATM relying on an Adaptive Clock method to smooth the network jitter. However, there is needed a method and apparatus to transfer variable bit rate (VBR) services across an ATM network as well as other types of networks. The approach to transporting variable bit-rate (VBR) services has not been fully defined. The main problem is in the recovery of the variable bit rate service clock. The suggested approaches to date involve explicit source clock state references being passed through the ATM network to a point where the service clock is being recovered. This information is used by a phase lock loop at that point to adaptively recover the clock by tuning the phase lock loop to the clock state references. However, such methods have not provided a satisfactory solution. A desired solution is one which does not involve the complexity of having to pass source clock state references through the ATM network. In a paper published by Guido Franceschini on Apr. 20, 1994 and distributed to the ATM Forum SAA Sub working group, an adaptive clock involving a 1 bit wide first-in first-out buffer (FIFO) and a phase lock loop was described as an appropriate method for an adaptive clock recovery method for a continuous bit rate stream. However, the difficulty with applying this approach to VBR service transport is that the cell rate across an arbitrary sequential group of cells can vary to such an extent that the phase lock loop circuit would not be able to lock. To get around this problem the latter reference suggested transferring the measure of the inter arrival times (at the sending side) with reference to an absolute counter implying that such a circuit would not work without transferring such information.
Accordingly it is an object of the invention to provide an improved method and apparatus for variable bit rate service clock recovery. It is a further object of the invention to provide a recovery method and apparatus for a variable bit rate service clock which does not require transferring rate information from the sending end.